1. Field of the Invention
The invention relates to an optical bench and to an optoelectronic package for housing at least an active optoelectronic device and its associated electronic circuit elements. In particular, the invention concerns a low cost thermally controlled optoelectronic package.
2. Related Art
In high-power laser devices, the laser unit, laser diode or laser diode chip, is generally mounted on a heat-sink made of a material with high thermal conductivity, such as diamond, SiC or AlN, for thermal dissipation during operation of the laser. The heat-sink is bonded onto a circuit board, which can accommodate all or part of the complementary internal devices of the laser package, such as a thermistor, a back-field photodetector and circuit patterns. Optical components, in particular optical fibres and lenses, are fixed in optically aligned position in front of the laser with the help of holding and aligning members (e.g., ferrule, saddle, clip). Holding and aligning members (herein also generally referred to as optical members) for fibres and lenses are fixed commonly by laser welding to a welding platform. To avoid displacement shifts, optical members and welding platform are generally made of a material with low thermal expansion coefficient. To this purpose, Kovar (an iron alloy comprising cobalt and nickel), stainless steel or Invar are often preferred. The baseplate on which the optical members are placed is also referred to as optical bench. The assembly comprising the optical bench and the circuit board is also referred to as the optical assembly. The optical assembly mounting the laser is generally placed on a thermoelectric cooler (TEC) in order to stabilise the laser temperature during operation.
In article ‘Automated Fiber Attachment for 980 nm Pump Modules’ by P. Mueller and B. Valk, IEEE Electronic Components and Technology Conference 2000, a laser diode on a AlN submount, a thermistor, a photodiode and an ESD (electrostatic dissipation device) are soldered onto an AlOx-circuit board. The circuit board is placed on a baseplate, which is mounted on a thermoelectric cooler (TEC). A fibre with support tube is welded to a clip, which is fixed on the free side of the baseplate. Clip, weld platform and fibre support tube are made of Kovar.
In PCT patent application WO 97/05513 a laser diode is placed on a substrate of alumina, which carries also different connections and other electronic components. The circuit board is mounted onto a metallic baseplate, preferably made of an alloy of chromium-nickel, such as Kovar. Kovar is said to be preferred because of its thermal compatibility with alumina substrates.
As the optical bench needs to be placed on a TEC, Applicants have observed that using a material with poor thermal conductivity, such as Kovar, for the baseplate does not allow proper heat dissipation over the TEC top surface.
Article ‘Laser Diode Packaging Technology: 980 nm EDFA Pump Lasers for Telecommunication Applications’ by Mobarhan K. S. and Heyler R., that was available on Oct. 24, 2000, in the Internet at the URL address http://www.newport.com/Support/Application_Notes/APPNOTES3.pdf, describes an optical subassembly that is a miniature aluminium nitride based optical bench containing the laser chip, laser diode submount, photodiode, and various other components. The optical subassembly also includes a relatively large metal platform onto which all the fiber pigtailing components are welded.
With regard to the packages that house the laser unit, presently 14-pin butterfly packages have become standardised throughout the industry for laser packaging, e.g., for 980 nm pump laser devices or 920 nm multimode lasers for fibre lasers or Raman amplifiers, since they enable customers to source from more than one supplier. In a butterfly type package terminal pins extend from the side surfaces of the package to be substantially parallel to the optical bench/circuit board. Electrical connections between the internal devices and the terminal pins are performed through wires or the like. The design of butterfly packages with standard position of the pin-out constrains the internal construction of laser modules as the position of the electrode pads (circuit patterns) of the internal devices (laser, photodetector, ESD, etc.) should substantially face the corresponding terminal pin in order to avoid the presence of long flying wires inside the package. This has lead to the development of ceramic terminal feed-through's, which extend through the side surfaces of the package and comprise a ceramic block provided with metallised pads from inside to outside the package for electrical connection to terminal pins. Metallised pads on the ceramic block can be designed to bridge the distance between the electrode pad of an internal device and the corresponding terminal pins. Examples of butterfly laser packages having ceramic terminal feed-through are given in U.S. Pat. Nos. 5,963,695, 5,930,430, and 5,195,102.
FIG. 1 shows a conventional optical bench applicable to 980 nm pump laser packages. The optical bench 1 comprises a baseplate 2 made of aluminium nitride (AlN). A circuit board 3 made also of AlN is bonded to the baseplate 2, which acts as a rigid support for the circuit board and for a Kovar welding platform 6, which is a single mounting block. To the Kovar platform optical components, e.g., fibre or lenses, will be fixed, e.g., by laser welding, in optically aligned positions with the help of holding and aligning members (not shown). FIG. 1(b) shows the top view of the conventional optical bench. On the circuit board 3, the laser 5 placed on the heat-sink 4 is mounted, plus some of internal complementary devices: electrostatic discharge element (ESD) 9, a back-field detector 10 and a thermistor 12. The circuit board is provided with metallised tracks (circuit patterns) 19.
The circuit board and the Kovar platform are bonded to the baseplate 2 with Au/Sn solder 8. A heat-sink 4 is placed on the circuit board 3 as submount for laser chip 5. The heat-sink is made of SiC, a material with high thermal conductivity. The heat-sink 4 is soldered to the circuit board 3 by Pb/Sn solder 11.
FIG. 2 shows the plan view of the 14-pin butterfly package 20 housing the optical bench of FIG. 1. Two ceramic terminal feed-through's 15 for the pin-out are provided with metallised pads 13 for connecting the internal devices (TEC, laser, thermistor, etc.) to the corresponding external terminal pins 14. Shape and length of the metallised pads are designed so as to shorten the distance between the electrode pads of the devices and the corresponding pins. Wires 16 connect the electrode pads of the internal devices to the metallised pads 13 of the ceramic feed-through's 15. The dotted-dashed line 22 indicates the optical axis of the optical bench (and of the package), i.e., the main emitting beam path of the laser to which the optical component, such as a fibre, is aligned. The baseplate 2 is mounted on a TEC, in this case a Peltier device (not visible).
Manufacture of packages with ceramic terminal feed-through for pin-out, as that described in FIG. 2, adds high costs to package fabrication, especially if compared to the seal-around technology. In seal-around technology, straight pins extend through apertures made on the side (butterfly type package) or on the bottom (dual in-line type package) surfaces of the package. Hermetic sealing, generally using glass or ceramic material, around the pins provides the means for sealing the pins to the package.
Applicants have observed that the seal-around technology in laser packages would be an attractive alternative to the ceramic terminal feed-through since it would lower manufacturing costs. They have remarked that an optical assembly (i.e., comprising the circuit board and the optical bench) according to FIGS. 1 and 2 is not suitable to packages that use seal-around technology. In said conventional optical assembly, the circuit board, provided with circuit patterns, is placed on a baseplate. In the longitudinal direction with respect to the package, i.e., in a direction substantially parallel to the optical axis 22, the circuit board extends only over a portion of the package. In a package with no ceramic feed-through, the use of such an assembly would then imply the presence of long electrical connections (wires or the like), thereby increasing the electrical inductance and the encumbrance inside the package.
Applicants have further observed that in the conventional optical assembly of FIG. 1, during laser operation, the heat produced by the laser spreads from the heat-sink to the circuit board and to the adjacent welding platform, it diffuses through the baseplate to reach finally the TEC top surface. In this way, heat dissipation does not occur over the whole TEC top surface as the welding platform, which is made of a material with poor thermal conductivity (e.g., Kovar), hinders efficient thermal dissipation in the direction essentially parallel to the TEC surface. Furthermore, heat spreading from the laser to the TEC surface has to follow a relatively long thermal path, i.e., through the heat-sink, the circuit board and finally the baseplate, thereby increasing the thermal resistance. The relatively large thermal resistance in the partially hindered heat flow to the TEC cause a loss in the TEC cooling efficiency with consequent increase in power consumption of the package.